汽車發(fā)動機內(nèi)燃機連桿加工工藝及夾具設計【帶CAD圖紙和說明書】

資源目錄里展示的全都有預覽可以查看的噢，，下載就有,,請放心下載，原稿可自行編輯修改=【QQ：11970985 可咨詢交流】====================喜歡就充值下載吧。。。資源目錄里展示的全都有，，下載后全都有,,請放心下載，原稿可自行編輯修改=【QQ：197216396 可咨詢交流】==================== 英文原文 Connecting rod In a reciprocating piston engine, the connecting rod or conrod connects the piston to the crank or crankshaft. The connecting rod was invented sometime between 1174 and 1200 when a Muslim inventor, engineer and craftsman named al-Jazari built five machines to pump water for the kings of the Turkish Artuqid dynasty — one of which incorporated the connecting rod. Transferring rotary motion to reciprocating motion was made possible by connecting the crankshaft to the connecting rod, which was described in the "Book of Knowledge of Ingenious Mechanical Devices". The double-acting reciprocating piston pump was the first machine to offer automatic motion, but its mechanisms and others such as the cam, would also help initiate the Industrial Revolution. Internal combustion engines In modern automotive internal combustion engines, the connecting rods are most usually made of steel for production engines, but can be made of aluminium (for lightness and the ability to absorb high impact at the expense of durability) or titanium (for a combination of strength and lightness at the expense of affordability) for high performance engines, or of cast iron for applications such as motor scooters. They are not rigidly fixed at either end, so that the angle between the connecting rod and the piston can change as the rod moves up and down and rotates around the crankshaft. The small end attaches to the piston pin, gudgeon pin (the usual British term) or wrist pin, which is currently most often press fit into the conrod but can swivel in the piston, a "floating wrist pin" design.The big end connects to the bearing journal on the crank throw, running on replaceable bearing shells accessible via the con rod bolts which hold the bearing "cap" onto the big end; typically there is a pinhole bored through the bearing and the big end of the con rod so that pressurized lubricating motor oil squirts out onto the thrust side of the cylinder wall to lubricate the travel of the pistons and piston rings. The con rod is under tremendous stress from the reciprocating load represented by the piston, actually stretching and relaxing with every rotation, and the load increases rapidly with increasing engine speed. Failure of a connecting rod, usually called "throwing a rod" is one of the most common causes of catastrophic engine failure in cars, frequently putting the broken rod through the side of the crankcase and thereby rendering the engine irreparable; it can result from fatigue near a physical defect in the rod, lubrication failure in a bearing due to faulty maintenance, or from failure of the rod bolts from a defect, improper tightening, or re-use of already used (stressed) bolts where not recommended. Despite their frequent occurrence on televised competitive automobile events, such failures are quite rare on production cars during normal daily driving. This is because production auto parts have a much larger factor of safety, and often more systematic quality control. When building a high performance engine, great attention is paid to the con rods, eliminating stress risers by such techniques as grinding the edges of the rod to a smooth radius, shot peening to induce compressive surface stresses (to prevent crack initiation), balancing all con rod/piston assemblies to the same weight and Magnafluxing to reveal otherwise invisible small cracks which would cause the rod to fail under stress. In addition, great care is taken to torque the con rod bolts to the exact value specified; often these bolts must be replaced rather than reused. The big end of the rod is fabricated as a unit and cut or cracked in two to establish precision fit around the big end bearing shell. Therefore, the big end "caps" are not interchangeable between con rods, and when rebuilding an engine, care must be taken to ensure that the caps of the different con rods are not mixed up. Both the con rod and its bearing cap are usually embossed with the corresponding position number in the engine block. Recent engines such as the Ford 4.6 liter engine and the Chrysler 2.0 liter engine, have connecting rods made using powder metallurgy, which allows more precise control of size and weight with less machining and less excess mass to be machined off for balancing. The cap is then separated from the rod by a fracturing process, which results in an uneven mating surface due to the grain of the powdered metal. This ensures that upon reassembly, the cap will be perfectly positioned with respect to the rod, compared to the minor misalignments which can occur if the mating surfaces are both flat. A major source of engine wear is the sideways force exerted on the piston through the con rod by the crankshaft, which typically wears the cylinder into an oval cross-section rather than circular, making it impossible for piston rings to correctly seal against the cylinder walls. Geometrically, it can be seen that longer con rods will reduce the amount of this sideways force, and therefore lead to longer engine life. However, for a given engine block, the sum of the length of the con rod plus the piston stroke is a fixed number, determined by the fixed distance between the crankshaft axis and the top of the cylinder block where the cylinder head fastens; thus, for a given cylinder block longer stroke, giving greater engine displacement and power, requires a shorter connecting rod (or a piston with smaller compression height), resulting in accelerated cylinder wear. Compound rods Many-cylinder multi-bank engines such as a V-12 layout have little space available for that many connecting rod journals on a limited length of crankshaft. This is a difficult compromise to solve and its consequence has often led to engines being regarded as failures. The simplest solution, almost universal in road car engines, is to use simple rods. This requires the rod bearings to be narrower, increasing bearing load and the risk of failure in a high-performance engine. This also means the opposing cylinders are not exactly in line with each other. In certain types of engine, the master rod carries one or more ring pins to which are bolted the much smaller big ends of slave rods on other cylinders. Radial engines typically have a master rod for one cylinder and slave rods for all the other cylinders in the same bank. Certain designs of V engines use a master/slave rod for each pair of opposite cylinders. A drawback of this is that the stroke of the subsidiary rod is slightly shorter than the master, which increases vibration in a vee engine. The usual solution for high-performance aero-engines is a "forked" connecting rod. One rod is split in two at the big end and the other is thinned to fit into this fork. The journal is still shared between cylinders. The Rolls-Royce Merlin used this style. Crank linkage of the type and characteristics The use of the internal combustion engine crank linkage of many types, according to kinematics perspective can be divided into three categories, namely: Heart crank linkage, the eccentric crank linkage and the main vice-link crank linkage. Centre crank linkage is characterized by the cylinder through the centerline of the crankshaft rotation centre and perpendicular to the axis of rotation of the crank. This type of linkage in the internal combustion engine crank in the most widely used. The single-engine general, tied for linkage with the use of the V-shaped chaxing link the internal combustion engine, and the home of the piston internal combustion engine crank linkage fall into this category. Eccentric crank linkage is characterized by vertical cylinder centerline of the crankshaft rotating in the center, but not by crankshaft rotary centre, the cylinder centerline distance between the crankshaft with a rotary axis offset e. This crank linkage institutions can reduce the swelling in the itinerary of the piston and cylinder intramural largest lateral pressure so that the pistons in the expansion programme and pressure reduction programme in the cylinder wall at the role of lateral pressure on both sides of the relatively uniform size. Vice-link the main crank linkage is characterized by: the internal combustion engine cylinder with a main link, the other out vice-link cylinder used, these are not direct link to the bottom of the crank pins, but on sale through the deputy link with in the main link of the big heads, formed a "joint" movement, such institutions also sometimes referred to as "joint song stalk linkage ".Crank linkage in the joint, a crank can put a few of connecting rod and piston, This structure will shorten the length of the internal combustion engine, compact and widely used in high-power locomotives used tanks and V-shaped internal combustion engine. Boring In machining, boring is the process of enlarging a hole that has already been drilled (or cast), by means of a single-point cutting tool (or of a boring head containing several such tools), for example as in boring a cannon barrel. Boring is used to achieve greater accuracy of the diameter of a hole, and can be used to cut a tapered hole. The term boring is also sometimes used for drilling a hole. Machine Boring The boring process can be carried out on a lathe for smaller operations, but for larger production pieces a special boring mill (work piece rotation around a vertical axis) or a horizontal boring machine (rotation around horizontal axis) are used. A tapered hole can also be made by swiveling the head. The boring machines (similar to the milling machines such as the classic Van Norman) come in a large variety of sizes and styles. Work piece diameters are commonly 1-4m (3-12 ft) but can be as large as 20m (60ft). Power requirements can be as much as 200 hp. The control systems can be computer-based, allowing for automation and increased consistency. Because boring is meant to decrease the product tolerances on pre-existing holes, several design considerations must be made. First, large length-to-bore-diameters are not preferred due to cutting tool deflection. Next, through holes are preferred over blind holes (holes that do not traverse the thickness of the work piece). Interrupted internal working surfaces—where the cutting tool and surface have discontinuous contact—should be avoided. The boring bar is the protruding arm of the machine that holds cutting tool(s), and must be very rigid. Forging Forging is the term for shaping metal by using localized compressive forces. Cold forging is done at room temperature or near room temperature. Hot forging is done at a high temperature, which makes metal easier to shape and less likely to fracture. Warm forging is done at intermediate temperature between room temperature and hot forging temperatures. Forged parts can range in weight from less than a kilogram to 170 metric tons.Forged parts usually require further processing to achieve a finished part. The?selection?of?base?and?positioning? The?base?position?is?protocol?machining?line,?determine?the?processing?plan?of?the?important?work?to?do?first? Make.?Choose?correctly,?the?yankees?will?directly?influence?the?reasonable?or?not,?the?machining?of?quality?and?productivity.? When?selecting?position?in?the?plane,?need?to?consider?the?following?questions:? (1)?in?which?a?surface?as?the?essence?and?surface?or?unified?standards,?(2)?to?ensure?machining?precision,?(3)?Make?the?machining?process?smoothly?? (4)?for?processing?the?pure?plane?or?unity,?(5)?should?be?adopted?as?a?rough?surface?plane??(6)?whether?individual?processes?for?special?processing?requirements,?(7)?need?to?employ?unified?standards?of?plane?outside?? According?to?the?essence?and?the?yankees?choice:?fine,?fine?and?selection?principles?should?be?considered?when?the?first,?the?benchmark?overlap?In?may,?namely,?under?the?situation?of?the?machined?surface?should?be?chosen?for?the?locating?datum?design?standards.?In?the?process?of?connecting?the?machining?process,?most?of?a?specified?process?selection?of?end?and?small?head?hole?As?the?main?base,?with?big?hole?in?the?side?outside?the?designated?as?another?round?face?includes?surface.?This?is?due?to?the?face.??Large?area,?more?stable,?With?little?head?position?can?directly?control?size?hole?hole?center?distance?of?the?head.?It?makes?The?process?of?locating?datum,?reduce?positioning?error.? Further?study:?again,? According?to?the?drawing,?the?size?of?the?connecting?parts?have?a?head?hole?surface?(namely?the?right?end)?in?a?flat?surface,? Therefore,?but?can?also?be?processed?another?end?(left)?is?not?an?end,?for?the?plane?as? It?is?disadvantageous?position?datum.?Therefore,?in?the?process?of?the?localization?plan?to?idea. As?the?yankees?round?hole?small?head,?so?little?head?outside?surface?processing?round?hole?arrangement?early.?In?small?head?and?large? Before?the?yankees?head?hole?as?orientation?of?the?machining?processes?and?drilling,?reaming,?these?procedures?for?processing?after?the?hole?The?hole?is?parallel?with?that?sometimes?affects?the?processing?precision?follow-up?processes,?therefore,?should?be?in?processing?Note?that?chooses?reasonable?positioning?scheme.? In?the?procedure?of?the?workpiece?in?each?surface?is?blank?surface,?positioning?and?clamping?conditions?are?poor,?but? Limits.but?and?cutting?force?is?bigger,?if?it?meets?the?workpiece,?the?rigid?differential?itself?for?machining?accuracy?can?have?very? Big?effect,?therefore?the?procedure?of?the?positioning?and?clamping?the?choice?of?methods,?for?the?whole?process?of?processing? Degree?often?far-reaching?effects.?Processing?is.?Connecting?In?rough?milling?of?how?the?workpiece?position??A?method?is?In?the?end,?with?blank?side?and?end?milling?a?clamping,?rough?surface,?with?good?surface?milling,?positioning.? Another?blank,?milling.?But?because?the?blank?surface?smooth,?connecting?rod,?the?rigid?localization?clamping?workpiece. May,?after?the?rough?surface?deformation?milling,?a?smooth?seems?to?relax,?workpiece?and?restore?distortion,?influence?subsequent?processes?Positioning?accuracy. 中文譯文 連 桿 在活塞往復式發(fā)動機內(nèi)，連桿連接著裝在曲柄或曲軸上的活塞?！肚擅畹臋C械裝置知識》一書這樣寫道：“連桿發(fā)明于1174年至1200年的某個時候，當一個名為阿拉-賈扎里的穆斯林發(fā)明家、工程師和工匠，制造了5個機器來為土耳其阿爾圖格王朝的一位國王泵水——這些機器的其中之一就使用了連桿。將旋轉運動轉變成往復運動可能需要依靠連接到曲柄上的連桿?！彪p作用往復活塞泵是第一個提供自動運動的機器，但其機構和其他如凸輪一類的機構也有助于工業(yè)革命的開啟。 內(nèi)燃機 在現(xiàn)代汽車內(nèi)燃發(fā)動機里，用于發(fā)動機的連桿通常由鋼制造，但也可以用鋁（目的是為了減輕重量和獲得在犧牲耐久度的條件下吸收強沖擊的能力）或鈦（目的是為了在需要支持力時提供一種既輕又有足夠強度的組合）來制造高性能發(fā)動機的連桿，或使用鑄鐵，如制造摩托車連桿時就使用鑄件。它們不會嚴格地固定于一端，于是當連桿作上下運動和繞曲柄旋轉時連桿與活塞之間的夾角就發(fā)生改變。 連桿較小的一端連接活塞銷，活塞銷（英國用語）或腕銷，這通常會給連桿以經(jīng)常性的壓力，但連桿仍能相對于活塞轉動即“浮動腕銷”。連桿的大端連接于曲柄上的軸頸處，并隨著由連桿螺栓固定的可更換的軸瓦轉動，螺栓將軸承“蓋”固定在連桿的大端處;通常要鉆一個通過軸瓦和連桿的大端小孔，以便使增壓潤滑油能噴到筒壁的一側，來使活塞和活塞環(huán)的運動得到潤滑。 連桿承受著巨大的壓力，這些壓力來自于由活塞產(chǎn)生的循環(huán)載荷，而事實上這些壓力來自于每次旋轉時的拉伸與松弛，以及隨發(fā)動機轉速增大而急劇增大的載荷。一個失效的連桿，通常被稱為“扔棒”，它是引起汽車引擎災難性故障最常見的原因之一，經(jīng)常使失效的連桿穿過曲軸軸箱的一側，發(fā)動機會遭受無法彌補的損壞；它可能源于連桿的疲勞缺陷、軸瓦失去潤滑而導致的失效，或源于連桿螺栓的缺陷、不適當?shù)木o固，或重復利用已經(jīng)使用過的（已變形的）螺栓（這是不允許的）。盡管這些經(jīng)常發(fā)生在競爭激烈的汽車運動中，但在為日常駕駛生產(chǎn)的汽車中，這種失效是十分罕見。這是因為汽車零部件的生產(chǎn)中要使用一個比較大的安全系數(shù)，同時往往還使用更系統(tǒng)的質(zhì)量控制體系。 當制造一個高性能發(fā)動機時，連桿應給予極大的關注，應采取一些技術來消除應力，例如磨削連桿的邊緣以達到表面粗糙度的要求，噴丸以使表面產(chǎn)生壓應力（防止裂紋萌生），裝配時平衡所有連桿、活塞組合件的重量使每對的重量相同以及采用磁力探傷法來探測材料內(nèi)部的微小裂紋，這些看不見的微小裂縫將會產(chǎn)生破壞應力造成連桿失效。此外，扭緊連桿螺栓時，應非常注意扭矩的大小；通常這些螺栓必須更換，而不是重復使用。連桿的大端被制造成一個整體，并使其在機械加工之后能與大端軸瓦準確裝配。因此，大端的“帽子”在連桿與連桿之間不具有互換性，而且當重新制造一個引擎時，必須小心，以確保不同連桿的軸瓦不被亂用。無論是連桿還是與其相配合的軸瓦，通常都會在發(fā)動機缸體上刻上相應的型號。 目前有一些發(fā)動機（如福特的4.6升引擎，還比如克萊斯勒的2.0升引擎）其連桿采用粉末冶金技術制造，粉末冶金技術不僅能精確控制尺寸和重量以減少機械加工工作量而且還能減少額外的機械配平。軸瓦因擠壓而與連桿分離，結果導致了不平滑的斷裂面，這是由于粉末金屬的顆粒造成的。這確保了重新裝配后，軸瓦能與連桿精確地配合，而傳統(tǒng)加工方法制造的連桿與軸瓦，只有當兩者的接觸面表面的表面粗糙度都很小時才能達到較小的誤差。 發(fā)動機磨損的一個主要原因是由于曲軸通過連桿施加于活塞的側向力，通常將汽缸磨成橢圓形截面，而不是圓形截面，因此不可能使活塞環(huán)與氣缸側壁緊密接觸。從力學角度來說延長連桿的長度可相應地減少上述側向力，這樣一來會使引擎壽命延長。然而，對一已知的發(fā)動機缸體來說，連桿的長度加上活塞行程，其和是一個固定的值，這個固定值由曲軸和氣缸座（氣缸座用來固定活塞蓋）頂部之間的固定距離來決定。因此，對一個已知的氣缸而言能得到更長的行程，可提供更大的排量和功率。相反，較短的連桿（或較小壓縮行程的活塞），會導致氣缸加速地磨損。 復合連桿 眾多多缸布局的發(fā)動機——如V – 12型發(fā)動機——幾乎沒有可用于在有限長度的曲軸上安裝連桿軸頸的空間。這是一個難以調(diào)和的矛盾，而且若按普通的方式安裝，其往往會導致發(fā)動機的失敗。 最簡單的解決辦法是使用簡單的連桿，這種最簡單的方法通常用于汽車引擎。這就要求連桿軸瓦要更窄，但對于一個高性能的引擎來說其會增加軸瓦的負荷及失效的風險。這也意味著對置的氣缸不完全位于一條直線上。 在某些類型的引擎內(nèi)，主動連桿帶有一個或多個環(huán)形銷，環(huán)形銷用來連接其他氣缸上的從動連桿相對小一些的大端。徑向引擎的每一邊通常是一個氣缸有一個主動連桿，余下的其它氣缸則配有從動連桿。對于確定設計的V形引擎，一對對置的氣缸使用一對主/從動連桿。這樣的一個缺點是，輔助連桿的行程稍微短于主動連桿，從而使V形發(fā)動機產(chǎn)生更大的振動。 高性能航空發(fā)動機的通常解決方案是使用一個“叉狀”連桿。一個連桿在大端處一分為二，另一個變薄以與這個叉狀連桿相配。軸頸仍然由多個氣缸共用。勞斯萊斯默林發(fā)動機就使用這種形式。 曲柄連桿機構的類型及特點 內(nèi)燃機中采用曲柄連桿機構的型式很多，按運動學觀點可分為三類，即:中心曲柄連桿機構、偏心曲柄連桿機構和主副連桿式曲柄連桿機構。中心曲柄連桿機構的特點是氣缸中心線通過曲軸的旋轉中心，并垂直于曲柄的回轉軸線。這種型式的曲柄連桿機構在內(nèi)燃機中應用最為廣泛。一般的單列式內(nèi)燃機，采用并列連桿與叉形連桿的V形內(nèi)燃機，以及對置式活塞內(nèi)燃機的曲柄連桿機構都屬于這一類。 偏心曲柄連桿機構的特點是氣缸中心線垂直于曲軸的回轉中心線，但不通過曲軸的回轉中心，氣缸中心線距離曲軸的回轉軸線具有一偏移量e。這種曲柄連桿機構可以減小膨脹行程中活塞與氣缸壁間的最大側壓力，使活塞在膨脹行程與壓縮行程時作用在氣缸壁兩側的側壓力大小比較均勻。主從連桿式曲柄連桿機構的特點是:內(nèi)燃機的一列氣缸用主動連桿，其它各列氣缸則用從動連桿，這些連桿的下端不是直接接在曲柄銷上，而是通過從動連桿銷裝在主連桿的大端上，形成了“關節(jié)式”運動，所以這種機構有時也稱為“關節(jié)曲柄連桿機構”。在關節(jié)曲柄連桿機構中，一個曲柄可以同時套上幾副連桿和活塞，這種結構可使內(nèi)燃機長度縮短，結構緊湊，廣泛地應用于大功率的坦克和機車用 V形內(nèi)燃機。 鏜 在機械加工，鏜削加工的過程是一個擴孔的過程，這個孔可以是鉆出來的（或鑄造得到的），鏜孔通過單點切削刀具（或一個鏜頭含有若干個這樣的刀具）來加工，例如用鏜削方法加工炮桶。鏜削加工能使孔達到更精確的尺寸，而且還可以用于錐形孔的加工。 鏜削有時也用于孔的加工。 鏜孔機 對較小的鏜削加工過程可以在車床上進行，但對于較大工件的加工則需要使用特殊的鏜床（工件圍繞一個垂直軸旋轉）或臥式鏜床（圍繞水平軸旋轉）。通過轉動變換刀具的安裝角度也可以加工錐形孔。 鏜床（類似于銑床，如經(jīng)典的范諾曼型）擁有多種尺寸和類型。工件直徑通常是1 – 4米（ 3-12英尺） ，但也可達20米（六十英尺）。對電力的需求可高達200匹馬力。其控制系統(tǒng)可以以計算機為基礎，允許自動控制和提高一體性。 由于鏜削加工可以降低產(chǎn)品上已有孔的公差，因此一些設計的注意事項必須得注意。首先，大的長徑比是不希望的，因為這樣會使刀具變形。其次，不能加工盲孔（孔的深度不超過工件的厚度）。中斷的內(nèi)部工作表面（即在刀具與加工表面間有不連續(xù)的接觸）應該避免。裝有刀頭的鏜桿是一個懸臂梁，必須有非常高的剛度。 鍛造 鍛造是一種利用局部壓力使金屬成型的方法。冷鍛是在室溫下或接近室溫下進行的鍛造。熱鍛是在高溫下進行，高溫使金屬更容易成形和降低斷裂的可能性。溫鍛是在室溫和熱鍛溫度之間的溫度下進行。鍛造可對從不足1千克到170噸的工件進行加工。經(jīng)鍛造加工的零部件通常還需作進一步處理，以便得到最終的產(chǎn)品。 度下進行。鍛造可對從不足1千克到170噸的工件進行加工。經(jīng)鍛造加工的零部件通常還需作進一步處理，以便得到最終的產(chǎn)品。 定位基面的選擇? 定位基面的選擇是擬定零件的機械加工路線，確定加工方案中首先要做的重要工作。基面選擇得正確、合理與否，將直接影響工件的加工質(zhì)量和生產(chǎn)率。?在選擇定位基面時，需要同時考慮以下問題：?(1)?以哪一個表面作為加工時的精基面或統(tǒng)一基準，(2)?才能保證加工精度，(3)?使整個機械加工工藝過程順利地進行?? (4)?為加工上述精基面或統(tǒng)一基準，(5)?應采用哪一個表面作為粗基面??(6)?是否有個別工序為了特殊的加工要求，(7)?需要采用統(tǒng)一基準以外的精基面?? 精基面的選擇：根據(jù)精基面的選擇原則，選擇精基面時，首先應考慮基準重合的問題，即在可能的情況下，應盡量選擇加工表面的設計基準為定位基準。??在連桿機械加工工藝過程中，大部分工序選用連桿的一個指定的端面和小頭孔作為主要基面，并用大頭孔處指定一側的外圓面作為另一基面。這是由于：端面的面積大，定位比較穩(wěn)定；用小頭孔定位可直接控制大小頭孔的中心距。這樣就使各工序的定位基準統(tǒng)一起來，減少定位誤差。?再深入研究一下：? 根據(jù)零件圖，連桿零件的大小頭孔有一端面（即右端面）在一個平?面上，因此可以同時加工出來，但另一端面（即左端面）不在一個平面上，這對作為定位基準面來說是不利的。因此，在加工時需主意其定位方案的合理性。? 小頭孔外圓作為基面，所以小頭孔外圓表面的加工安排得比較早。?在小頭孔與大頭孔作為定位基面前的加工工序且鉆孔、擴孔、鉸孔，這些工序對于加工后的孔與孔的平行度不易保證，有時會影響到后續(xù)工序的加工精度，因此，在加工時應注意，選用合理的定位方案。? 在第一道工序中，工件的各個表面都是毛坯表面，定位和夾緊的條件都較差，而加工余量和切削力都較大，如果再遇上工件本身的剛性差，則對加工精度會有很大影響，因此第一道工序的定位和夾緊方法的選擇，對于整個工藝過程的加工精度常有深遠的影響。連桿加工就是如此。在粗銑中工件如何定位呢？一個方法是以毛坯端面定位，在側面和端部夾緊，粗銑一個端面后，翻身，以銑好的面定位，銑另一個毛坯面。但是由于毛坯表面?不平整，連桿的剛性差，定位夾緊時工件可能變形，粗銑后端面似乎平整了，一放松，工件又恢復變形，影響后續(xù)工序的定位精度。